Grinding machine



y 14, 1963 l R. s. HAHN 3,089,288

GRINDING MACHINE Filed March 22, 1960 v e 3/ 23 /6 4/ 25 l I I I 1 V 47 4 24 Y 45 Y K m 55 ';E5z 59 44 5 R 2'7 I as 1g 5 43 57 68 I INVENTOR.

Robert S. Hahn United States Delaware Filed Mar. 22, 1960, Ser. No. 16,889 11 Claims. (CI. 5150) This invention relates to a grinding machine and more particularly to apparatus for finishing metal surfaces by abrasion.

In the art of grinding and, especially in the art of internal grinding, it is desirable to finish the surfaces of the workpiece as rapidly as possible. This is primarily true because the capital cost of the machine continues twenty-four hours a day, so that the greater the amount of work accomplished on the machine, the less the cost of the capital overhead which must be applied to each workpiece operation. There are several ways in which a grinding operation may be speeded up; first of all, the grinding rate may be increased. This must be done without loss of accuracy of dimension or of finished surface quality. At the same time, the number of large high inertia ele ments in the machine may be reduced to a minimum, or the amount of movement to which such heavy elements are subjected may be reduced. One of the difi'iculties experienced in the past, for instance, is that in order to reciprocate the grinding wheel over the work surface during the grinding operation, it is necessary not only to move the whole wheelhead, but also the table on which the wheelhead is mounted and the motor which drives the wheelhead. This large mass of equipment has very great inertia and severely limits the rate of reciprocation, and, of course, increases the sizes of the hydraulic cylinders which must be used to bring about this reciprocation. Furthermore, according to the prior art, the reciprocation of the elements associated with the wheel is accomplished by a large hydraulic cylinder, the flow of fluid to which is controlled by valves which, inturn, are controlled by dogs and latches which are mounted on the workhead table and the base of the machine. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the present invention to provide a grinding machine in which wheel reciprocation takes place without the necessity of reciprocating large high inertia elements of the machine.

Another object of this invention is the provision of a grinding machine having a wheelhead with a built-in means for reciprocating the wheel and wheel spindle independently of the wheelhead.

A further object of the present invention is the provision of a grinding machine having a simplified fluid system for producing axial reciprocation of the wheel and spindle.

It is another object of the instant invention to provide a grinding machine having a wheel head in which the spindle is held in such a manner that it has a very high spring constant or rigidity.

It is a further object of the invention to provide a grinding machine wheelhead in which the spring constant of the spindle is very high, in which the grinding Wheel pressure possible is higher than previously and in which the surface finish is greatly improved despite the use of higher grinding forces.

' atent ice A still further object of this invention is the provision of a grinding machine in which the reciprocation of the wheel takes place without the use of complicated dogs, latches, and hydraulic valving.

It is a still further object of the present invention to provide a grinding machine in which the abrasive Wheel reciprocation takes place without movement of the wheelhead, the wheelhead drive motor, or the wheelhead table.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

FIG. 1 is a schematic front elevational view of a grinding machine embodying the principles of the present invention;

FIG. 2 is a vertical sectional View of a portion of the grinding machine; 7

FIG. 3 is a schematic view of a hydraulic circuit associated with the invention;

FIG. 4 is a graph showing certain fluid pressure relationships which take place during the operation of the machine; and

FIG. 5 is a sectional view taken on the line VV of FIG. 2.

Referring first to FIG. 1, wherein are best shown the general features of the invention, the grinding machine, designated generally by the reference numeral 10, is shown in use finishing an internal bore in an annular workpiece 11. The workpiece 11 is mounted for rotation on a workhead 12 which, in turn, is mounted on a base 13. The workhead 12 is mounted on the base in such a manner that it is capable of movement transversely of the axis of the workpiece and such movement for feeding the workpiece takes place by means of a conventional feeding mechanism 14. Also mounted on the base is a dresser 20 of the usual type having a diamond. At the other end of the base 13 is mounted a wheelhead table 15, which, in the present invention, generally remains fixed to the top of the base and does not move during a grinding machine operation. On top of the table 15 is mounted a wheelhead 16 and its motor 17. A similar motor 18 is mounted on the workhead 12 and is connected to the workhead to provide a driving force for the workpiece 11 in the usual manner. Extending from the wheelhead 16 is a spindle 26 on which is mounted an abrasive wheel 21.

Referring now to FIG. 2, which shows a vertical crosssectional view of the wheelhead 16, it can be seen that a wheelhead housing '22 is provided with a broad, fiat base for attachment to the upper surface of the table 15 and the upper portion has a generally elongated cylindrical form which is provided with :a bore 23 extending entirely through the housing. Mounted in the end of the housing 23 which faces away from the workpiece 11, is a bearing assembly 24 and at the other end of the bore 23 is mounted a bearing assembly 25. Extending through both bearing assemblies is a spindle 26. At one end the spindle is provided with a threaded bore 27 adapted to receive a holding screw for the wheel 21, while at the other end it is provided with a threaded bore 28 adapted to receive a pulley which provides a driving rotation of the spindle by a belt extending from the motor 17 Of course, other drivin means may be provided, such as an aligned highfrequency electric motor. At its central portion the spindle 26 is provided with a circular flange 29. A piston 31 is slidably mounted in the bore 23 between the bearing assembly 24 and the bearing assembly 25. The piston 3-1 and the bore 23, therefore, constitute an actuator which may be operated by a fluid such as air or oil. The piston 31 is provided with a counter-bore 32 which is concentric with the bore 23 and the shaft 26 and which enters the piston 31 from the side facing the wheel end of the spindle. The flange 29 resides in this bore in the piston and the spindle 26passes through the head of the piston through a bore 33; The bearing assembly 25 is provided with a boss 34 whichis concentric with the spindle and which extends toward the piston 31 and into the bore 32 a short distance. Around the boss 34 is placed a coil spring'35 which, at one end, presses against the bearing assembly 25 and resides in a shallow recess 36; at the other end, the coil spring 35 presses against a sleeve 37 which fits loosely within the bore 32 in the'piston and also fits loosely around the flange 29. A substantial distance exists between the sleeve 37 and the boss 34 so that the piston 31 may move a substantial distance within the bore 23 against the spring compression. The inner end of the bearing assembly "24 entirely fills the bore 23 and cooperates with the end of the piston 31 to form a fluid chamber 3-8,. should be noted that the piston 31 is provided with a suitable keyway 39 which slides along a key 41 fastened in the wall of the bore 23. The bearing assembly 24 is provided with a flange 42 at its outer end by which means it is bolted to the housing 22. It is provided with a bore 43 in which the spindle 26 resides and the bore is provided with shallow annular recesses 44 and 45. The recess 44 is connected to a passage 46 which meets an orifice member 47 which is screwed into the body of the bearing assembly and whose passage opens into an annulargroove '48 formed on the outer surface of the bearing assembly and defining a fluid chamber with the wall of the bore 23. The ends of the groove 48 are suitably sealed by rubber O-rings. A passage '49 extends through the housing 22 from its base to the surface of the bore 23 within the groove 48 and is suitably connected to a source of high fluid'pressure (not shown). The recess 45 is also connected by means of a passage 51 through a restriction, such as the orifice member 52, to the groove 48. Fluid oil collection grooves 53, 54, and 55 are formed in the surface of the bore 43 of the bearing assembly 24 and are joined by suitable passages to a fluid collection manifold 56'which, in turn, is connected to the sump of the high pressure fluid system (not shown). A suitable seal member 57 is formed as part of the bearing assembly 24 and surrounds the shaft 26 to prevent leakage of fluid therearound.

The bearing assembly 25 is provided with two shallow recesses 58 and 59. Each recess has the'general configuration shown in FIG. 5, wherein three pockets are separated by'lands which are adapted to lie fairly close to the surface of the spindle. A fluid-admitting passage enters the recesses at the pockets. The recess 58 is connected through a passage 61 and an orifice member 62 to an annular groove 63 formed on the outer surface of the bearing assembly 25 and defining with the bore 23 of the housing an oil chamber. In a similar manner, the re cess 59 is connected through a passage 64 and an orifice member 65 to the groove 63. The groove 63 is connected to the source of high pressure fluid mentioned above. The groove 63 also is provided with sealing O-rings. One end of the bearing assembly 25 is provided with an oil collection groove 66 and between the recesses 58 and 59 with an oil collection groove 67, these two grooves being connected by a passage and, in turn, connected to the oil collection manifold '56.

Referring now to FIG. 3, which shows a schematic view of the fluid system, it can be seen that the chamber 38 is connected to a liquid pressure circuit. Liquid in the chamber 38 will, of course, press against the piston 31 Itf against the pressure of the spring '35. The chamber 38 is connected by a fluid line to a three-way valve 71. The valve is connected by a line 72 to a check-valve 73, which, in turn, is connected through a pressure regulating valve 74 to a pressure fluid source (not shown). The checkvalve is arranged so that it opens only when the pressure in the line 72 is less than the pressure atthe output of the pressure regulating valve 74. The line 72 is also connected through a three-way valve 75 to a line 76 leading to a reciprocating plunger 77 which may be actuated by a motor driving it through a Scotch-yoke mechanism 78. The valve 75 is also connected to a third line 79' leading to the atmosphere. The three-wayvalve 71 is also conected .to al-ine 851 leading to a three-way valve 82 which, in turn, is connected in one position to atmosphere through a line 83 and in another position to atmosphere through a line 84 and a flow restriction 85.

The operation of the invention'will now be readily under-stood in view of the above description.v Pressure fluid passing through the pressure regulating valve 74 will be received by the circuit 68 at constant pressure. The check-valve 73 :assures that there will be no feeding of pressure fluid from the line 72 back through the pressure regulating valve. The pressure coming through the pressure regulating valve 74 will be static and at a constant value. This fluid will pass through the line 68 and the valve 71 to the chamber 38 where it will press against the piston 31 and cause it to travel to the left against the compression force or bias of the coil spring 35. Since the fluid pressure coming through the valve 74 is at a constant value, the piston will deflect the spring a certain amount until a position is reached at which the fluid pressure is balanced by the spring pressure; the piston will remain in that position so long as the pressure exists against it. Upon the actuation of the Scotch-yoke mechanism 78 by a motor, the plunger 77 will reciprocate and cause fluid pressure pulsations to pass through the line 76 and the valve 75 to the line 72. These pulsations will be superimposed upon the pressure from the valve 74 and will pass into the chamber 38 where they will be felt. FIG. 4 shows the situation which will exist because of the air pressure from these two sources. The pressure labeled P (see FIG. 4) is the pressure coming through the valve 74 and is a straight line; that is to say,'its value is constant'with time. The pressure indicated by the letter -P, representsthe pulsations emitted by the plunger 77 and these will be superimposed on the pressure P When the plunger 77 is under compression, the pulsations will add to the pressure in the line 72. The resulting pressure appearing in the chamber 38 will be an alternating pressure pattern P superimposed on a bias pressure P,,. This will cause the piston 31 to move back and forth from the position to which it was moved by the primary fluid pressure controlled by the valve 74. The piston will move back and forth and carry the spindle 26 with it, but the magnitude of the reciprocations will be fairly small. If it is desired to have the piston remain in the fixed deflected position, -it is only necessary to rotate the valve 75 so that the pulsations from the plunger 77 pass through the line 79 to atmosphere. The pressure in the chamber 38 then will be P only. If it is desired to return the piston 31 to its original position of rest, it is only necessary to remove the fluid from the chamber 38 and, for that purpose, the three-way valve 71 is rotated to cause the fluid to flow from the chamber 38 into the line 81. With the three-way valve 82 in the position shown, the fluid will then pass into the line 84 and through the restriction 85 to atmosphere. The restriction will cause the fluid to flow slowly and the piston will move slowly back to its original position. At times it is necessary that the piston be moved quickly to its original position and, for that purpose, the valve '82 is rotated so that the fluid passes from the line 81 into the line 83 and'to atmosphere. The internal diameter of the lines is sufficient to assure that there is no appreciable restriction to the flow of the fluid from the chamber and that the piston moves quickly to its original position. In the operation of the grinding machine shown in FIG. 1, the workpiece 11 is mounted on the workhead 12 and is rotated by means of the motor 18. The wheel 21 is introduced into the bore of the workpiece and the workpiece is fed transversely of the Wheel axis by means of the feed mechanism 14. During the grinding operation, the valve 75 is in the position shown in FIG. 3 and the pattern of pressures will be the resultant pressures shown in FIG. 4. If the wheel is in a position removed from the workpiece, as shown in FIG. 1, it is first necessary to introduce it into the bore of the workpiece. This is done by the actuation of valve 71 so that it connects the line 72 with the line 69. The pressure P controlled by the valve 74 will pass into the chamber 38 and deflect the piston 31 to the left carrying the spindle 26 and the wheel 21 with it into the bore of the workpiece. Once the wheel 21 is in the bore of the workpiece and the feed mechanism 14 has moved it toward the surface to be finished, the valve 75 is rotated so that the pressure pulsations from the plunger 77 will be introduced into the line 72 and will cause the piston 31 and the spindle 26 to reciprocate, thus reciprocating the wheel 21 in the workpiece as it rotates. According to the conventional grinding cycle, the wheel 21 must be removed from the workpiece and dressed by means of the dresser and its diamond before being introduced into the bore for a finish operation. For that purpose the valve 71 is rotated so that it connects the lines 69 and 81. The valve 82 is in the position shown in FIG. 3, that is to say, joining the line 81 to the line 84, so that the fluid is admitted to the chamber 38 but must pass through the orifice restriction 85 with reduced volume. The piston 31, therefore, moves to the right slowly, carrying the spindle and wheel with it. The wheel passes over the diamond of the dresser 20 and is dressed at the necessary slow speed. Then the diamond is moved out of the way and the pressure controlled by the valve 74 is re-introduced into the chamber 38 by rotating the valve 71 and the piston 31 again moves to the left while pulsations from the pump 77 are introduced so that the wheel reciprocates in the work for the finish grind. When the finish grind is over, it is necessary to retract the wheel quickly out of the way in order to load a new workpiece into the machine. For that purpose, the valve 71 is rotated so that it connects the lines 69 and 81, while the valve 82 is rotated so that it connects the lines 81 and 8 3. The fluid has a free path from the chamber 38 so that the piston 31 moves quickly to the right carrying the spindle and wheel with it.

It has been noted that the spindle 26 is rotated by means of the motor 17. Fluid is introduced into the wheelhead 16 through the passage 49 from a pressure fluid source. It flows into the grooves 48 and 63 of the bearing assemblies 24 and respectively. The fluid in the groove 48 flows through the orifice member 47 and the passage 46 to the recess 44 surrounding the spindle 26. Similarly, fluid passes from the groove 48 through the orifice member 52 and the passage 51 to the recess 45 by passages not shown in the drawings. The same fluid from the passage 49 passes into the groove 63 in the bearing assembly 25. This fluid from the groove 63 passes through the orifice member 62 and passage 61 into the recess 58 and also through the orifice member 65 through the passage 64 into the recess 59. This static pressure fluid completely surrounds the spindle in the projected areas of the recesses. It should be noted that the recesses 44, 45, 58 and 59 extend along the length of the spindle a considerable distance so that this pressure fluid against the surface of the spindle provides an excellent bearing. At the same time, it makes the spindle very stiff; the fluid in the bearings reduces its spring constant. Any attempt to bend the spindle by means of external forces and also any attempt to vibrate the spindle will be prevented by the hydrostatic bearings. It can be seen that any vibrations will be dampened by the fluid pressure in the recesses. Any attempt to establish modes of vibration along the spindle will be resisted by the fluid surrounding the spindle and situated along the vibration-damping lands. The fluid in the recesses has a tendency to leak since it is at high pressure. The leaking fluid moves axially along the spindle, is collected in the collector grooves 53, 54, 55, 66, and 67, and is carried through suitable passages to the fluid collection manifold 66 from which it is returned to the sump of the high pressure fluid source.

Referring to FIG. 5, which shows a section through one of the bearings, the recesses surrounding the shaft have a plurality of relatively deep pockets with a corresponding number of adjoining lands where the fit between the shaft and the diamete genera-ted by the lands is extremely close. These land areas act as force-cushioning devices because of the viscosity of the fluid present between the shaft and the bear-ing. When a force is applied transversely to the axis of the shaft, the natural tendency is for the fluid to travel away from the line or area on that shaft which is diametrically opposite to the point of force application at which the force is applied. The present design not only impedes that natural tendency because of the small passages provided in the surrounding manifold for the supplying of fluid to the pockets and the lands, but also provides local viscous dampening in the land areas to neutralize motion of the shaft due to sudden loading and unloading of transversely applied forces.

It can be seen that the use of the present invention offers the possibility of significant simplification in production grinding machine designs. The invention makes possible the design of machines wherein, instead of reciprocating the heavy table with the heavy workhead and work-head motor, it is only necessary to reciprocate the spindle. The elimination of the table, table-ways, reverse dogs and levers, pilot valves, reverse valves, piston, cylinder, and the way-lubrication system results in lower manufacturing costs and reduced maintenance. Such a hydrostatic wheelhead in addition to offering compact and efiicient design possesses superior grinding qualities as were shown by tests. During these tests, grinds were made at various forces and the rate of stock removal and surface finish were measured comparing a conventional ball bearing wheelhead with the present hydrostatic bearing type wheelhead of the invention. It can be seen that the surface finish is generally lower for the present hydrostatic wheelhead. In addition, when the softest wheel was used, 40 lbs. was the maximum force permissible on a ball bearing head of conventional design without causing wheel breakdown, whereas a 50 lb. force was permissible for the same wheel on the wheelhead of the present invention. The spring constant (stiffness) at the wheel for the ball bearing head was 90,000 lbs. per inch, while with the hydrostatic head it was 120,000 lbs. per inch. The visual appearance of the finished surface from the point of view of chatter and finish was much better when using the hydrostatic head. It was quite clear that the differences in behavior were due to a great extent to the better vibration characteristics of the present wheelhead, as explained in connection with FIG. 5.

While certain novel features of the invention have been shown and described and are pointedout in the annexed claims, it will be understood that various omissions, substitutions and changes in the forms and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

l. A grinding machine, comprising a wheelhead including an axially-reciprocable spindle, a resilient member biasing the spindle in one direction, a source of static pressure connected to the spindle to move it in the other direction against the said bias, and means for introducing to the said static pressure a series of pressure pulsations to bring about axial reciprocation of the spindle.

2. A grinding machine, comprising .a wheelhead including an axially-reciprocable spindle, a resilient member biasing the spindle in onedirection, a source of fluid under static pressure connected to the spindle to move it in the other'direction against the said bias, the source including :a pressure regulating valve and a check-valve arranged to allow flow of fluid only from the source to the spindle, a pump introducing to the said air between the check-valve and the spindle a series of fluid pressure pulsations to bring about axial reciprocation of the spindle, and means for selectively disconnecting the pump and the source from the spindle. 1

3. A grinding machine, comprising a base, a wo-rkhead mounted on the base, a wheelhead which is mounted on the base and which carries an axially reciprocable spindle, means for bringing about a feeding movement between the workhead and the wheelhead transversely of the axis of the spindle, a resilient member biasing the spindle in one direction, a fluid-operated actuator, associated with the spindle to bring about the said axial movement, a source of static fluid pressure connected to the actuator to move the spindle in the other direction against the said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

4! A grinding machine, comprising a wheelhead having a bore therein, a spindle extending into the bore and slidable therein in an axial direction, a resilient member biasing the spindle in one'direction, a fluid-operated actuator associated with the spindle, a source of static fluid pressure connected to the actuator to move the spindle in the other direction against the-said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

5. A grinding machine, comprising a wheelhead having a bore therein, a spindle lying in the bore for rotation therein and for axial reciprocation, a resilient member biasing the spindle in one direction, two sets of bearings in the bore, the bearings carrying the spindle and being spaced axially thereof, each bearing having a set of shallow recesses lying close to and facing the surface of the spindle, means including flow restrictions for supplying fluid under pressure to each recess, a source of static Ifluid pressure connected to the spindle to move it in the other direction against the said bias, and means introduc'ing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

6. A grinding machine, comprising a base, a workhead mounted on the base, a wheelhead which is mounted on the base, and which carries a spindle, means for bringing about a feeding movement between the workhead and the wheelhead transversely of the axis of the spindle, the wheelhead having a bore therein, the spindle extending into the bore and slidable therein in an axial direction, a resilient member biasing the spindle in one direction, a fluid-operated actuator associated with the spindle to bring about the said axial movement, two sets of hearings in the bore, the bearings carrying the spindle and being spaced axially thereof, each bearing having a set of shallow recesses lying close to and spaced from the surface of the spindle, means including a flow restriction for supplying 'fluid under pressure to each recess, and a source of fluid pressure pulsations connected to the actuator to cause the said axial movement.

7. A grinding machine, comprising a wheelhead having a bore therein, a, spindle lying in the bore for rotation therein, the spindle being also axially slidable in the bore, aresilient member biasing the spindle in one direction, a

fluid-operated actuator associated with the spindle to cause the said movement in the other direction, two sets of bearings in the bore, the bearings carrying the spindle and being spaced axially thereof, each bearing having a set of shallow recesses lying close to and facing the surface of the spindle, means including a flow restriction for supplying fluid under pressure to each recess, a source of static fluid pressure connected to the actuator to move the 8'; spindle in the otherdirection against the said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bringabout axial reciprocation of the spindle.

8. A grinding machine, comprising a base, a workhead mounted on the base, a wheelhead which is mounted-on the base and which carriesa spindle, meansfor bringingabout a feeding movement between the workhead and the wheelhead transversely of the axis of the spindle, the spindle being capable of reciprocating axial movement Within the wheelhead, a fluid-operated actuator associated with the spindle to bring about the said axial movement, a resilient member biasing the spindle in one direction a source of static fluid pressure connected to the spindle to move it in the other direction against the said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

9. A grinding machine, comprising a base, a workhead mounted on the base, a wheelhead which is mounted on the base and which carries a spindle, means for bringing about a feeding movement-between the workhead and the wheelhead in a direction transverse to the axis of the spindle, the wheelhead having'a bore therein, the spindle lying in the bore for rotation therein and for reciprocating an axial movement therein, two sets of bearings in the bore, the bearings carrying the spindle and being spaced axially thereof, each bearing having a set of shallow recesses lying close to and facing the surface of the spindle, and means including a flow restriction for supplying fluid under pressure to each recess, a resilient'member biasing the spindle in one direction, a fluid-operated actuator associated with the spindle to bring about the said'axial movement, a source of static fluid pressure connected to the actuator to move the spindle in the other direction against the said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

10. A grinding machine, comprising a base, a workhead mounted on the base, a wheelhead which is mounted on the base and which carries a spindle, means for bringing about a feeding movement between the workhead and the wheelhead transversely of the axis of the spindle, the wheelhead having a bore therein, the spindle extending into the bore and sli-dable therein in an axial direction, a resilient member biasing the spindle in one direction, two sets of bearings in the bore, the bearings carrying the spindle and being spaced axially thereof, each bearing having a set of shallow recesses lying close to and'facing the surface of the spindle, and means including a flow restriction for supplying fluid under pressure to each recess, a fluid-operated actuator associated with the spindle to bring about the said axial movement, a source of static pressure connected to the actuator to move the spindle in the other direction against the said bias, and means introducing to the said pressure a series of fluid pressure pulsations to bring about axial reciprocation of the spindle.

11. An internal grinding machine for finishing the surface of a bore in a workpiece, comprising a base, a workhead mounted on the base and adapted to carry the workpiece for rotation about the axis of thebore, a wheelhead which is mounted on the base and which carries a spindle and an abrasive wheel for rotation therein, the wheelhead having a bore therein, andithe spindle extending into the bore and slidable therein in an axial direction, means for bringing about a feeding movement between the workhead and the wheelhead in a direction transverse to the axis of the spindle to feed the wheel into the surface of the bore, a coil spring lying in the bore and biasing the spindle in one direction, a fluid-operated piston concentric with and connected to the spindle to cause the said move .ment in the other direction, two sets of hearings in the 3,089,288 V 9 16 set of shallow recesses lying close to and extending enfor selectively disconnecting the pump and the source from tirely around the spindle, and means including a flow the spindle. restriction for supplying hydraulic fluid under pressure to each recess, a source of fluid under static pressure References Cited in the file Of this Patent connected to the piston to move it in the other direction 5 UNITED STATES PATENTS against the said bias, the source including a pressure regulating valve and a check valve arranged to allow flow of 1164134 stamgl et 1915 2,270,586 Jahant et al Ian. 20, 1942 fluid only from the source to the plston, a pump introducin to th said f uid a series of pressure pulsations to 2661580 Cmmpton 1953 2,839,874 Maker June 24, 1958 bring about axial reciprocation of the spindle, and means 

1. A GRINDING MACHINE, COMPRISING A WHEELHEAD INCLUDING AN AXIALLY-RECIPROCABLE SPINDLE, A RESILIENT MEMBER BIASING THE SPINDLE IN ONE DIRECTION, A SOURCE OF STATIC PRESSURE CONNECTED TO THE SPINDLE TO MOVE IT IN THE OTHER DIRECTION AGAINST THE SAID BIAS, AND MEANS FOR INTRODUCING TO THE SAID STATIC PRESSURE A SERIES OF PRESSURE PULSATIONS TO BRING ABOUT AXIAL RECIPROCATION OF THE SPINDLE. 